Names+and+Formulas+for+Ionic+Compounds

The simplest ratio of the ions represented in an ionic compound is called a formula unit. The overall charge of any formula unit is zero. In order to write a correct formula unit, one must know the charge of each ion. The charges of monatomic ions, or ions containing only one atom, can often be determined by referring to the periodic table or table of common ions based on group number. For example, ions of group 1A typically have a charge of 1+. Those of group 2A have a charge of 2+. Those of group 7A have a charge of 1-.

The charge of a monatomic ion is equal to its oxidation number. The oxidation number, or oxidation state, of an ion in an ionic compound is numerically equal to the number of electrons that were transferred to or from an atom of the element in forming the compound. If the electrons were tranferred from the atom, the ion has a positive oxidation state. If they were transferred to the atom, the ion has a negative oxidation state. Most transition metals and groups 3A and 4A have more than one oxidation number.

Oxidation numbers can be used to determine the chemical formulas for ionic compounds. If the oxidation number of each ion is multiplied by the number of that ion present in a formula unit, and then the results are added, the sum must be zero.

In the formula for an ionic compound, the symbol of the cation is written before that of the anion. Subscripts, or small numbers written to the lower right of the chemical symbols, show the numbers of ions of each type present in a formula unit.

Example: Determine the correct chemical formula for the ionic compound formed between strontium (Sr) and bromine (Br).

Strontium is an element in group 2A of the periodic table. Bromine is in group 7A. The formulas for their ions are therefore Sr2+ and Br1-, respectively. Sr must lose two electrons to form its ion, and Br must gain one electron. The total number of electrons lost must equal the total number of electrons gained so that overall charge is zero. Subscripts must be chosen so that this is the case. For electrical neutrality, there must be two Br for each Sr. The correct formula is thus SrBr2, where the subscript 1 is understood after the Sr. This formula can be verified by multiplying the subscripts by the ion charges and summing the result, as follows. (1 x 2+) + (2 x 1-) = 0

An ion that contains more than one atom is called a polyatomic ion. The charge on such an ion applies to the entire group of atoms. The writing of chemical formulas for compounds containing such ions follows the same rules as for compounds containing only monatomic ions. The overall charge must be zero. Subscripts within the formula for a polyatomic ion must not be changed during formula writing. If there is more than one such ion is a formula unit, parentheses are written around the formula of the ion and a subscript is written after the final parenthesis.
 * Polyatomic Ions**

Example: Write the chemical formula for a compound formed from ammonium ions and sulfate ions. Refer to a table of common ions as necessary.

The ammonium ion has the formula NH4+. The sulfate ion has the formula SO4 2-. The overall charge of the formula unit must be zero, which means that there must be two ammonium ions for each sulfate ion. Thus the correct formula is (NH4)2SO4. The subscript after the parenthesis indicates the presence of two ammonium ions.

In naming ionic compounds, name the cation first, then the anion. Monatomic cations use the element name. Monatomic anions use the root of the element name plus the suffix -//ide//. If an element can have more than one oxidation number, use a Roman numeral in parentheses after the element name, for example, iron (II) to indicate the Fe 2+ ion. for polyatomic ions, use the name of the ion.
 * Naming Ionic Compounds**

Certain polyatomic ions, called oxyanions, contain oxygen and another element. If two different oxyanions can be formed by an element, the suffix -ate is used for the oxyanion containing more oxygen atoms, and the suffix -ite for the oxyanion containing fewer oxygens. In the case of the oxyanions of the halogens, the following special rules are used.

four oxygens, per + root + -ate (example: perchlorate, ClO4 1-) three oxygens, root + -ate (example: chlorate, ClO3 1-) two oxygens, root + -ite (example: chlorite, ClO2 1-) one oxygen, hypo- + root + -ite (example: hypochlorite, ClO 1-)